1. Field of the Invention
The present invention relates to a color copying machine and, more particularly, to a color copying machine which is capable of achieving excellent color balance and which is also capable of reproducing a copy image of an original with high precision.
2. Description of the Prior Art
In conventional color copying machines, since the surface potential of a photosensitive body on which an electrostatic latent image is to be formed changes over time, it is difficult to obtain copy images of stable color balance over a long period of time.
This problem will be described briefly with reference to the gradation reproduction characteristics of color copies shown in FIG. 1, wherein Do represents the original image density, Dp represents the printed image density, E is the light exposure, and Vs is the surface potential of the photosensitive body. The first quadrant represents the gradation reproduction characteristics of the gray scale as an original. Curves C, M and Y respectively represent the gradation reproduction characteristics of copy images of cyan, magenta and yellow, and C+M+Y represents the gradation reproduction characteristics of copy images of mixtures of these three colors. The second quadrant (clockwise from the first quadrant) represents the exposure characteristics of Do-E (common logarithm). The third quadrant represents the color separation electrostatic latent image characteristics of red (R), green (G), and blue (B), respectively. The fourth quadrant represents the developing characteristics of images with toners of cyan (C), magenta (M), and yellow (Y), respectively.
The printed image densities Dp of cyan, magenta, and yellow are different from the image densities Do of the same original in the characteristics of the first quadrant, since unnecessary light absorption of the respective toners is corrected to obtain the desired color balance. Since the gray balance is involved as the color balance in this case, unnecessary light absorption of blue and green by the cyan toner, and unnecessary light absorption of blue by the magenta toner are corrected to achieve the desired color balance of an image of the mixture of cyan, magenta and yellow. For this purpose, the color separation electrostatic latent image characteristics of red, green and blue are made substantially the same as shown in the third quadrant, and the developing characteristics of cyan, magenta, and yellow are made different as shown in the fourth quadrant.
In order to obtain a copy image of good color balance, it is necessary to keep the color separation electrostatic latent image characteristics of red, green and blue constant for a long period of time. This is extremely difficult to achieve with conventional color copying machines as described above. The reason for this is attributable to changes in the environment such as temperature and humidity, degradation in the quality of the photosensitive body, and the resultant fluctuations in the electrostatic latent image characteristics. It is also necessary to correct the variations in the developing characteristics of the toners of cyan, magenta and yellow according to the manufacturing lots, and variations in the developing characteristics of these toners which occur over time. Considering this, it has been the general practice to intentionally make the color separation electrostatic image characteristics of red, green and blue as shown in the third quadrant deviate from one another.
However, it is extremely difficult to control the color separation electrostatic latent image characteristics of the respective colors. Specifically, a skilled operator must control the output of the charger of the color copying machine, the light exposure or the like using a surface potentiometer and a recorder. Therefore, the general user is unable to perform this control.